1. Field of the Invention
This invention relates to compositions of matter useful for machine washing of dishes, glassware, and the like, and it also relates to methods of washing which utilize a composition of the kind hereinafter described for washing with water not highly heated.
2. Description of the Prior Art
U.S. Pat. No. 3,812,045, issued May 21, 1974, accurately teaches: "Detergent compositions for use in automatic dishwashers must meet a number of criteria such as protection of overglaze decoration of china, non-spotting of glassware, non-tarnishing of silverware, detergency for cleaning the items being washed, absence of filming, non-caking of composition on the shelf, non-gelling of the composition in the washing machine, retention of available chlorine during shelf life for subsequently sanitizing items in the washer and others. All too frequently the solution to one problem results in the recurrence of one of the other difficulties."
It may be taken as a part of the prior art that there has been used, as taught in volume 19 of the Chemical Formulary by Bennett, a formulaton consisting of, by weight, 3 percent of "Pluronic 24R2" nonionic surfactant made by BASF Wyandotte Corporation, 35 percent of tetrasodium pyrophosphate, 20 percent of sodium polyphosphate, 10 percent of sodium metasilicate pentahydrate, 2 percent of chlorinated cyanurate, 18 percent sodium carbonate, and 12 percent water. In general, except for some possible changes in respect to the identity and proportion of the nonionic surfactant used, and the inclusion in the surfactant of a minor proportion of monostearyl acid phosphate as a defoaming agent as taught in U.S. Pat. No. 3,314,891, such detergents have been the best ones currently commercially available, but they are not phosphatefree.
U.S. Pat. No. 3,899,436 discusses the problem of obtaining a machine-dishwashing detergent of low phosphate content, indicating that much, but not all, of the phosphates should be replaced with sodium citrate or with citric acid, assuming that enough of alkali-metal hydroxide is present to neutralize the acid.
Moreover, the prior art does not, for the most part, concern the additional problem of finding a dishwashing detergent that will work properly when the wash water is relatively cooler than what has been used before. Ever since the remarkable increase in the price of oil in 1973, it has been evident that it would be desirable to have a detergent which performs adequately in cooler water, but it has been clear to those skilled in the art that it would be difficult to find a composition that would perform satisfactorily at lower wash-water temperatures, both because higher temperatures make the oils and fats on the dishware or glassware to be cleaned less viscous and more removable and because the nonionic surfactants used in dishwasher-detergent compositions, like other non-ionic surfactants, generally give greater amounts of foam with lower water temperatures.
It is known, moreover, that the problem with foam in the use of a dishwasher detergent is made particularly more difficult because of the tendency, unless something is done, for the nonionic surfactant to react with proteinaceous material, such as egg soil, to yield especially large proportions of foam--foam of a kind which is particularly difficult to reduce or disperse. U.S. Pat. No. 3,314,891 teaches the idea of including 0.1 to 50 weight percent of stearyl acid phosphate or oleyl acid phosphate, together with 50 to 99.9 weight percent of nonionic surfactant, as the nonionic-surfactant component in the dishwashing-detergent composition, as a way of overcoming the problem of foaming caused by egg soil, but this patent obviously provides no answer to those skilled in the art if a phosphate-free detergent is required.
The general idea of producing a nonionic surfactant by starting with a material having a plurality of active hydrogen compounds and then reacting it, first with ethylene oxide, to produce a plurality of oxyethylene units on the sites of the active-hydrogen compound, and then reacting the material so obtained further with propylene oxide, to produce polyoxypropylene capping chains which are connected to the oxyethylene chains, is disclosed in U.S. Pat. No. 3,036,118. U.S. Pat. No. 2,979,528 discloses similar nitrogen-containing surfactants or detergents, but ones in which the oxypropylene units are added first and the oxyethylene units form a cap. Neither of the above-mentioned patents indicates, however, the possibility that with a tetrafunctional nitrogen-based nonionic surfactant meeting the specifications indicated hereinbelow, with interior oxyethylene blocks and exterior caps of oxypropylene units, it would be possible, in the dishwasher-detergent art, to obtain a detergent which is not only phosphate-free but also effective, even against long-hardened partially cooked egg soil, when the wash water is at a low temperature, such as 100.degree. F. (about 38.degree. C.) or even lower.
In the art of formulating dishwasher-detergent compositions, it has been usual to make a distinction between compositions intended for home use--ones containing 0 to 3 percent of nonionic surfactant--and ones intended for institutional or commercial use--ones containing 2 to 6 percent of nonionic surfactant. The former ones are usually less alkaline, and they are intended to use in machines having a cycle on the order of 40 to 60 minutes, and the latter ones are compositions which are relatively more alkaline and are intended for use in machines which have a shorter cycle, one on the order of 15 to 20 minutes.
With the nonionic surfactants which have been commercially available, it has usually been impossible to obtain satisfactory dishwashing performance if the composition or formulation contains anything more than about 5 or 8 percent of "filler material", such as sodium chloride or sodium sulfate; usually, it has been necessary, in order to obtain satisfactory performance, to omit such materials altogether. To obtain satisfactory performance from a composition containing over 10 weight percent of filler material is, in view of the prior art as explained above, a surprising result.
The usual tests to which a dishwasher-detergent composition can be subjected may be taken as belonging to the prior art.
The formost among such tests is an egg-soil detergency test. Dinnerware is soiled with partially cooked egg, and permitted to stand for a certain period of time in air, and then washed in a dishwasher, usually using a washing liquid containing 0.3. weight percent of detergent and usually using conditions such as a wash at 150.degree. F. and a rinse at 180.degree. F., although in the case of efforts to develop a detergent effective at lower temperatures, other wash/rinse temperature conditions are used, such as 140/140 or 120/120. The number of minutes that such egg soil is permitted to stand, before dishwashing begins, has a distinct effect upon the percentage of removal of egg soil that is observed. Although many commercially available detergents perform very satisfactorily on such egg soil which has stood for only 10 minutes or less, especially when hot water (wash/rinse at 150/180) is used, the results with a standing time of as long as 17 minutes or 27 minutes are usually notably less satisfactory (5, 10, 20 percent not clean).
Another test commonly performed is a chlorine stability test. It is common to expect a detergent to have a shelf life of 3 or 4 months, and it is important that its active-chlorine content not decrease by too much over that length of time. A common accelerated test is to store the material for one month at 50.degree. C., determining the available chlorine before and after. The results are generally considered equivalent to those obtained with 4 months of storage at room temperature.
It can be taken as known that, in respect to protection of the overglaze decoration on china, the nonionic surfactant which is used has little effect; the principal effect comes from the combination of alkali-metal salts or other compounds (sodium carbonate, sodium silicate, sodium hydroxide, etc.) which are used.